Pitot tubes have long been used in the measurement of air flow. Pitots measure the total pressure and the static pressure, the difference therebetween being the square of the velocity pressure which is proportional to either the velocity of the pitot moving through the air (such as in avionics) or the square of the air velocity moving past the pitot (such as in ductwork, etc.).
The measurement of total pressure is relatively simple. On the surface of any body, regardless of shape, immersed in a stream of fluid there is some point (often the forward most point) at which the fluid is brought to rest and the pressure acting is the total pressure of the undisturbed flow. Thus, the pressure can be determined by providing an orifice at that point and connecting it to a manometer. This is the basis of the classic pitot tube which has been universally adapted for the measurement of total pressure.
Static pressure on the other hand, is comparatively difficult to obtain as the flow at the static orifices should be the same, both in speed and direction, as it was before the measuring instrument was introduced. Thus, in order to avoid vortices, the static pressure is normally measured from a point which is at least four diameters (but preferably more) away from the leading edge of the measurement device. Nevertheless, it is difficult to produce a pitot that accurately and reliably measures low fluid velocities (below 50 feet per minute) at low cost. In addition, developing a pitot that is accurate at very low velocities as well as high velocities has also proved difficult.
With the foregoing in mind, it is an object of the present invention to provide a pitot which measures very low fluid flow rates.
Another object of the invention is to provide a pitot which measures high fluid flow rates.
Still another object of the invention is to provide a single pitot that measures both low and high fluid flow rates.
Yet another object of the invention is to provide a pitot that is accurate.
A still further object of the invention is to provide a pitot that is inexpensive.
A still further object of the invention is to provide a pitot that is easy to fabricate.